THE ASTROPHYSICAL JOURNAL, 449:164-177, 1995 August 10

           VARIABLE STARS IN MAGELLANIC CLOUD CLUSTERS. II. NGC 1850

                           K. M. SEBO AND P. R. WOOD
          Mount Stromlo and Siding Spring Observatories, Private Bag,
                   Weston Creek PO, A.C.T. 2611, Australia;
                    kim@mso.anu.edu.au, wood@mso.anu.edu.au


                                   ABSTRACT

        We present the results of a study aimed at identifying variable
     stars in the rich LMC cluster NGC 1850.  V and I band CCD imaging
     spanning more than 5 years in a 10' x 10' field surrounding the
     cluster has allowed us to identify more than 30 variable stars,
     including seven classical Cepheids, one anomalous Cepheid, two RR
     Lyrae variables, 19 long-period variables (LPVs), one blue eclipsing
     binary, one possible pair of eclipsing giants, and several peculiar
     variables.  The only likely cluster member is a Cepheid.
        Isochrone fitting to the cluster CMD yields an age of 80 Myr if an
     LMC distance modulus of 18.5 is adopted.  A comparison of pulsation
     and evolution masses for the Cepheids still yields a ratio of
     evolution to pulsation mass of ~1.2 for evolution models computed
     with core overshoot parameter Lambda = 0.5, where Lambda = 2d_ov/H_p,
     d_ov is the distance that convection overshoots beyond the
     Schwarzschild boundary and H_p is the pressure scale height.  An
     overshoot parameter Lambda ~ 1.0 would be required to bring the
     evolution and pulsation masses into agreement.  One of the Cepheids in
     the field is a bump Cepheid.  A calculation of the bump mass yields a
     value in reasonable agreement with the pulsation mass.  We show that
     detailed modelling of individual bump Cepheids is capable of providing
     very tight constraints on the LMC distance modulus. The Cepheid
     period-luminosity-color relation compared to the theoretical relation
     yields a LMC distance modulus of 18.60.  Finally, arguments are
     presented which suggest that the LPVs are a mixture of fundamental
     mode pulsators and small-amplitude overtone pulsators.

     Subject headings: Cepheids -- Magellanic Clouds -- open clusters and
                       associations: individual (NGC 1850) -- stars:
                       evolution